In natural gas distribution systems requiring replacement of old metal piping, it has been found to be expedient to insert new plastic pipe into the old metal pipe to avoid having to dig up the old pipe. Such a situation occurs, for example, when replacing the section of pipe leading from a main line to a customer meter. Leakage in the old pipe system usually occurs in that section of metal pipe extending through the ground outside of a customer building. Within the building, exposure of plastic pipe is prohibited, so that it becomes necessary to provide a coupling between the plastic tubing inserted into the old pipe and metal distribution piping in the building leading to the meter. System-wide replacement of metal piping with inserted plastic pipe requires substantial standardization of the couplings used in connecting the plastic pipe to the usable metal pipe within the building and requires use of a coupling which has a limited length in order to fit easily within a limited space provided within the building at the point where the old metal pipe enters through the building and elbows upwardly, to the meter. For ease in installation, it is particularly desirable that one end of the coupling be sized to fit within the elbow before it turns upwardly and the other end be sized to thread onto the end of the old metal piping extending from the wall. In this way, the coupling may be merely inserted into the line without having to cut new threads or remove any portion of the old metal piping. However, it would be appreciated that the result of this procedure is to effectively increase the length of the pipe extending from the wall by an amount equal to the length of the coupling less whatever distances are threaded into the elbow and onto the end of the old metal piping. It will be appreciated that this increased length requires that the old metal piping extending upwardly from the elbow to the meter be bent further away from the wall. To avoid undue bending of the upright pipe and the possible introduction of stresses which may cause that pipe to leak, it clearly is desirable for the coupling to be as short as possible.
One prior form of coupling usable to connect a plastic pipe to a metal pipe is disclosed in U.S. Pat. No. 4,062,572. Another form of fitting usable for example in connecting together sections of plastic pipe is disclosed in U.S. Pat. No. 4,878,697 issued on Nov. 7, 1989. Still another form of coupling usable in connecting a plastic pipe to a metal pipe is disclosed in U.S. Pat. No. 3,375,025.